Jeff Bezos has pledged millions to the cause of same-sex marriage. Referendums held so far have not upheld marriage equality. Photograph: Evan Agostini/AP

Amazon founder Jeff Bezos and his wife MacKenzie have pledged $2.5m to supporters of gay marriage in Washington state, in what is believed to be the largest-ever single donation to the campaign for same-sex unions in the United States.

The donation doubles the amount of money available to supporters of a referendum in Washington, which seeks to uphold a same-sex marriage law that was signed by the state governor Chris Gregoire in February.

Jennifer Cast, a former Amazon employee who is a lesbian and the volunteer finance co-chair for Washington United for Marriage, emailed Jeff Bezos Sunday asking him to donate between $100,000 and $200,000 to the effort, writing:

I want to have the right to marry the love of my life and to let my children and grandchildren know their family is honored like a 'real' family. We need help from straight people. To be very frank, we need help from wealthy straight people who care about us and who want to help us win.

Cast received a reply Tuesday night, that said: "Jen, this is right for so many reasons. We're in for $2.5m. Jeff & MacKenzie."

In a statement, Washington United for Marriage said that the donation is believed to be the largest individual gift in the US towards the cause of same-sex marriage.

"The extraordinary contribution from Jeff and MacKenzie Bezos will make an enormous difference for our campaign to defend Washington's marriage law," said WUM's campaign manager, Zach Silk. "While it provides an amazing base for the work ahead, we hope it spurs others to invest because we're a long way from November and we face opponents with deep pockets who are committed to spending millions to defeat us."

For thousands of years, marriage between one man and one woman has proven to be the cornerstone of our society. Thirty-two other states have already voted to preserve marriage – November 6 is our opportunity to do the same. Since same-sex couples already enjoy all the benefits of marriage in Washington – and same-sex marriage would grant no new rights whatsoever – it's clear that the other side's intent in this battle is to reinvent the family and make marriage a genderless institution.

Though same-sex marriage is legal in six other states and Washington DC, each of the 32 ballot measures to legalize or ban same-sex marriage that have been put to vote have resulted in favor of those opposed to same-sex marriage.